Each lease shall be subject to the conditions of diligent development and continued operation of the mine or mines, except where operations under the lease are interrupted by strikes, the elements, or casualties not attributable to the lessee.

The Secretary of the Interior, upon determining that the public interest will be served thereby, may suspend the condition of continued operation upon the payment of advance royalties.

Advance royalties described in paragraph (2) shall be no less than the production royalty which would otherwise be paid and shall be computed on a fixed reserve to production ratio (determined by the Secretary).

The aggregate number of years during the period of any lease for which advance royalties may be accepted in lieu of the condition of continued operation shall not exceed 20 years.

 1 The amount of any production royalty paid for any year shall be reduced (but not below 0) by the amount of any advance royalties paid under a lease described in paragraph (5) to the extent that the advance royalties have not been used to reduce production royalties for a prior year.

 ¹ The Secretary may, upon six months’ notification to the lessee cease to accept advance royalties in lieu of the requirement of continued operation.

Nothing in this subsection shall be construed to affect the requirement contained in the second sentence of subsection (a) relating to commencement of production at the end of ten years.